Benzimidazole compounds, flubendazole and mebendazole, halt embrogenes in filarial worms in animal models and are now ready for testing in humans with onchocerciasis. They represent a novel mode of therapeutic action compared to diethylcarbamazine (DEC) which at the standard dose kills microfilaria en masse with frequent devastating side effects to the host. It is proposed to conduct an exciting pilot study to assess the safety and efficacy of these benzimidazoles compared with low-dose DEC therapy. Forthy-five men with moderate infection with onchocerciasis and without contraindication to treatment will be selected from an endemic area in Mexico. They will be entered into a double-masked clinical study and randomized to one of three treatment groups. One group will receive mebendazole, another flubendazole, and the third low-dose DEC. Medical ophthalmological, and parasitological examinations of the subjects, together with repeated fluorescein angiography, will be carried out. The study will be conducted in collaboration with Mexican investigators who will have primary patient responsibility. The applicants are experience investigators seeking immediate support for a pilot study to test these important, newly released compounds as soon as possible. The pilot study will determine the feasibility of setting up a more complicated and costly community-based study.